The present invention relates to a method for compensation of time variant field disturbances in magnetic fields of electromagnets with high field homogeneity, in particular in sample volumes of supra-conducting electromagnets for measurements of magnetic resonance, in said method the dispersion signal u.sub.x of the nuclear signal of a reference substance is acquired and used for compensation by generation of a current dependent on the dispersion signal in a field correction coil of the electromagnet.
In the known NMR field stabilization the dispersion signal u.sub.x of the nuclear resonance signal is used. In case of a deviation of the nuclear resonance signal from the nominal frequency the dispersion signal indicates immediately by its sign in which direction the compensation is to take effect.
The absorption signal u.sub.y is not used for the NMR field stabilization, the so-called NMR lock, but only for indication of the locked regulating state for the homogenization of the magnetic field by so-called "shimming". The absorption signal is known to be phase-shifted by 90.degree. compared to the dispersion signal.
One endeavors to carry out the NMR field stabilization as exactly as possible in order to maintain the magnetic field as exactly constant as possible. In doing so small disturbing amplitudes leading to a small frequency change of the reference substance, as well as relatively great external magnetic disturbance amplitudes are to be compensated. The invention provides a new method for the NMR field stabilization.